I have a Voltec three way adapter, it converts a 5-15 plug into a (3) 5-15 receptacles. Is it safe to use this adapter at either end of the extension cord? So three way adapter to extension cords then tools or extension cord to three way adapter then tools.
It really depends what you're powering
Three cell phone chargers, you're gonna be fine. But a table saw and a dust collector, ain't gonna happen.
The power-factor-corrected loads (i.e. Their VA not their watts) need to be added up. They need to be within the capacity of the circuit (15A=1800VA intermittent, or 12A=1440VA continuous). You should be fine if so.
Otherwise expect burnt and damaged splitters, and/or a breaker trip, depending on luck.
Presuming it is UL listed, it should safely work at its maximum rating. The biggest concern is that you can now have three high draw devices pulling power at once; you need to make sure this is never the case as you can over load the circuit. You hope the breaker trips, but you should double check the rating on the breaker as well on that electrical run.
The problem I have seen with these is multiple loads overheating the outlet and causing a melt down of the plug itself. The voltec is better than many but especially with cheap 98c outlets I have seen several failures on 15 amp protected circuits. I would want to use a quality heavy duty receptacle and still be cautious on the total load. They can be used at either end but make sure if at the end of an extension cord the cord is at least 14awg. The connections are where I have seen both outlets and extension cords fail.